1. Field of the Invention
This invention relates to a method and apparatus for controlling velocity and fuel/air ratio in premixed gas fired burners.
2. Description of the Prior Art
Premixed gas fired combustion is known to be a superior combustion mode for minimizing pollutant emissions, in particular, nitrogen oxides (NO.sub.x). However, known methods of premixing fuel and combustion air employ mixing far upstream of the burner nozzle, thereby creating a substantial risk of flame flashback, particularly, at low load operations, due to the nozzle velocity which decreases almost proportionally with load reduction.
Another known method of premixing fuel and air, known as nozzle mixing, involves mixing fuel and air within the nozzle. However, such methods cannot achieve perfect mixing due to the poor aerodynamic design and fixed cross-sectional area of the mixing channel in the nozzle. In addition, good mixing of the fuel and air at low load operation is not possible. Finally, known nozzle mixing methods do not permit the ratio of fuel and air to be adjusted within the nozzle to meet the requirements for turndown operation.
U.S. Pat. No. 1,702,298 teaches a burner for furnaces having a tubular casing with separate air and combustible gas inlets, the end of the burner casing facing the furnace being open, while the rear end of the casing is in enclosed by a cover having a tubular inward projection to which a threaded extension is attached. Disposed at the outlet end of the tubular projection having an opening through which the combustible gas flows is a tubular sliding piston which is moveable within the burner casing to control the flow of air and gas through the open end of the burner. Similarly, U.S. Pat. No. 4,755,136 teaches a burner for gaseous fuels in which two concentric tubular pistons control the introduction of gaseous fuel and air into a mixing chamber, the gaseous fuel being introduced through openings from the center of the inner piston into the mixing chamber and the air being introduced through openings in the surface of the outer piston into the mixing chamber.
A moveable insert for controlling the mixture of fuel and air within the nozzle of a burner is taught by U.S. Pat. No. 1,490,008 which discloses a burner for liquid fuel with a centrally inserted regulating piece having a conical end surrounded by an annular mixing chamber. U.S. Pat. No. 1,289,042 teaches a blow torch having a centrally positioned tube provided with a central passage connected to a gaseous fuel supply source, the tube having a plurality of discharge openings. Slidably mounted on the central gaseous fuel supply tube is a cylindrical valve, the movement of which regulates the amount of air flowing from an annular chamber around the centrally positioned tube as well as the amount of gaseous fuel from the centrally positioned tube through the discharge nozzle of the blow torch. Use of a moveable insert or plug in a mixing chamber area for regulating the flow of fluids therethrough is also taught by U.S. Pat. No. 3,721,387 which teaches an ejector having a variable primary nozzle area and mixing chamber area in which the plug in the mixing area varies not only the area of the primary nozzle but also changes the size of the length of the mixing chamber.
See also U.S. Pat. Nos. 1,434,238; 1,127,815; 1,667,365; 2,316,881; and 3,190,283, all of which relate to the combustion of oil, as opposed to a gaseous fuel, having means for adjusting the flow of air to control atomization of the oil and the firing rate of the burners. A premixed gaseous fuel fired burner with velocity and fuel/air ratio control is not taught by the known prior art.